1. Field of Invention
The present invention relates to an analogue buffer. More particularly, the present invention relates to a source-follow type analogue buffer using poly-Si TFTs for an active matrix display.
2. Description of Related Art
Low temperature poly-Si (LTPS) thin film transistors (TFTs) allow for peripheral integration of driving circuits with a pixel panel of an active matrix display due to a high current driving capability. However, it is well known that the integration of whole driving circuit with poly-Si TFTs is very difficult due to the rather poor characteristics and non-uniformity of poly-Si TFTs compared with single crystal Si large scale integrated circuits (LSIs). Among the driving circuits using poly-Si TFTs, analogue buffers are indispensable to drive the load capacitance of the data bus in the panel. Source follower is considered an excellent candidate for the analogue buffer circuit for the “System on Panel (SOP)” application because of its simplicity and low power dissipation.
A typical source follower 100 using a LTPS TFT in an active matrix display is shown in FIG. 1A. The gate of the TFT 110 in the source follower 100 coupled to a input voltage Vin and the drain of the TFT 110 is coupled to an operation voltage Vdd. The source of the TFT 110 is coupled to ground through a load capacitor (Cload). The waveform of output voltage Vout of the source follower 100 is depicted in FIG. 1B. It is observed that the final output voltage Vout is not kept constant, but exceeds the value of Vin-Vth expected in principle, where the Vth is a threshold voltage of the TFT 110. It is ascribed to the sub-threshold current. As shown in FIG. 1C, which depicts drain current (ID) and the voltage between gate and source of the TFT 110 (VGS) curves, the sub-threshold swing of LTPS TFTs is about 0.3V/dec which is much larger than that of a metal-oxide-semiconductor field effect transistor (MOSFET) (0.06V/dec). Consequently, the typical source follower 100, as an analogue buffer for active matrix display, will be sensitive to the charging time for various product specifications such as frame rates for the active matrix displays and can not have a constant output voltage.